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One way of getting this to work is by using the Locator as a Control of Manipulate.
For the convenience of easier spotting the replaced pixel I have cropped and magnified the image.
image = ExampleData[{"TestImage", "Lena"}];
img = ImageCrop[image, 100];
Manipulate[
Row@{img, Dynamic@(new = ReplacePixelValue[new, pt -> 0])} //
Magnify[#, 10] ...

Well I decided to give it a bit of a go...First import the image and convert to grayscale, then crop to focus on the area of interest. Then I used a LaplacianGaussianFilter, which is often used in blob detection.
img = ImageAdjust@ColorConvert[Import["http://i.imgur.com/4lDwE33.jpg"], "Grayscale"];
smallimg = ImageAdjust@ImageTake[img, {200, 500}, {200, ...

If ImageMeasurements didn't exist we could have used this one-liner:
Total[#]/Length[#] &@Flatten[ImageData[img], 1]
ImageData will give you a matrix of RGB vectors, Flatten[...,1] will then give you a one-dimensional list of RGB vectors. Total adds them together, by dividing by the number of RGB vectors we get the mean.
Also take a look at ...

I'm not 100% sure what you want: "segmentation" has a well-defined meaning in image processing, and I think that's not what you want. Also, I couldn't reproduce your results (I think DominantColors isn't guaranteed to give the same order or even the same results every time it's run). So this may or may not help...
First, this seems more reproducible than ...

This link talks about hacking a MagiQuest receiver with an Arduino. It mentions that the modulation rate of the IR signal is somewhere in the region of 37 kHz, a factor of more than 1000 times faster than anything your webcam can detect.
So you simply will not be able to do this with a webcam.
Even if the modulation rate were much lower you would still ...

Simple white balance
As described in the Wikipedia article, the simplest white balance is to rescale the RGB channels to make white objects have white pixels. Here's a simple method where I define a white point in the original image by the 0.995 quantile of each colour channel:
image = Import["http://i.stack.imgur.com/bUvXg.png"];
white = Quantile[#, ...

After you draw one or more selection rectangles and click away from the image they are still there suspended in a xenon mist but they are only visible if you look dead ahead use the selection tool. We can extract that data from the underlying Cell expression with this Button:
Button["Copy ImageMarkers",
Cases[
NotebookRead[SelectedNotebook[]],
...

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